1) SDXC - not natively, but you can format the micro SD to FAT32 to let the 3DS read it. I'm using an off brand 128 gb in my o3DS XL and it works well, but has slightly longer loading times. For me, the "sweet spot" between loading times and storage space is a 64 gb card. Lots of room for games and stuff, and less hassle loading the home menu.
2) Personally, I made the switch to the n3DS from the o3DS XL because I hated how bulky the "default" system was here. Seriously, the 3DS XL weighs a metaphorical tonne and you couldn't stuff it into a back pocket. I still don't use the 3D features (unless I'm playing a game like Super Mario 3D Land that requires me to use it.), the built in amiibo features, or the c-stick much (Why didn't the devs for DQ7 make the c-stick rotate the camera?).
Mostly I liked the design of the n3DS versus the extra processing power - and the ability to change faceplates. I'm mostly using it as an emulator/vc inject machine although. (Portable Earthbound? Yes!) Oh, and the Binding of Isaac as a portable? Yes...definitely.
3) Good luck selling your A9LH o3DS for $250. I got that price for an Animal Crossing one and a 64 gb card, charger and the out of print guide.